Toward solving the riddle: the enigma becomes less mysterious

Dabigatran Suppresses PAR-1/SphK/S1P Activation of Astrocytes in Experimental Autoimmune Encephalomyelitis Model

Multiple sclerosis (MS) is an inflammatory autoimmune disease affecting the central nervous system (CNS) that currently does not have any effective treatment. Experimental autoimmune encephalomyelitis (EAE) is often employed as a model to mimic the clinical manifestations of MS, mainly CNS demyelination. Coagulation is known to participate in crosstalk with inflammation and autoimmunity. We herein explored the correlation between the coagulation cascade and CNS immune diseases in vitro using primary astrocytes isolated from mice and in vivo using a mouse model of EAE. We showed that dabigatran, a clinical oral anti-coagulant drug, suppressed the thrombin-induced activation of astrocytes, and the underlying mechanisms are related to the activity of protease-activated receptor-1 (PAR-1), sphingosine-1-phosphate (S1P), and sphingosine kinases (SphKs). Importantly, dabigatran effectively recovered neurological function, reduced inflammation in the spinal cord, and prevented spinal cord demyelination caused by EAE. We suggest that dabigatran, a specific inhibitor of thrombin, antagonized the effect of thrombin in astrocytes by limiting the activation of PAR-1, in turn downregulating SphK1 and disrupting S1P receptor signaling. These findings reveal critical information about the relationship between coagulation mechanisms and CNS immune diseases and will contribute to the clinical translation and development of therapeutic strategies against MS.

Cardiovascular effects of sphingosine-1-phosphate (S1P)

Sphingosine-1-phosphate (S1P) regulates important functions in cardiac and vascular homeostasis. It has been implied to play causal roles in the pathogenesis of many cardiovascular disorders such as coronary artery disease, atherosclerosis, myocardial infarction, and heart failure. The majority of S1P in plasma is associated with high-density lipoproteins (HDL), and their S1P content has been shown to be responsible, at least in part, for several of the beneficial effects of HDL on cardiovascular risk. The attractiveness of S1P-based drugs for potential cardiovascular applications is increasing in the wake of the clinical approval of FTY720, but answers to important questions on the effects of S1P in cardiovascular biology and medicine must still be found. This chapter focuses on the current understanding of the role of S1P and its receptors in cardiovascular physiology, pathology, and disease.

Adipose tissue and ceramide biosynthesis in the pathogenesis of obesity

Although obesity is a complex metabolic disorder often associated with insulin resistance, hyperinsulinemia and Type 2 diabetes, as well as with accelerated atherosclerosis, the molecular changes in obesity that promote these disorders are not completely understood. Several mechanisms have been proposed to explain how increased adipose tissue mass affects whole body insulin resistance and cardiovascular risk. One theory is that increased adipose derived inflammatory cytokines induces a chronic inflammatory state that not only increases cardiovascular risk, but also antagonizes insulin signaling and mitochondrial function and thereby impair glucose hemostasis. Another suggests that lipid accumulation in nonadipose tissues not suited for fat storage leads to the buildup of bioactive lipids that inhibit insulin signaling and metabolism. Recent evidence demonstrates that sphingolipid metabolism is dysregulated in obesity and specific sphingolipids may provide a common pathway that link excess nutrients and inflammation to increased metabolic and cardiovascular risk. This chapter will focus primarily on the expression and regulation of adipose and plasma ceramide biosynthesis in obesity and, its potential contribution to the pathogenesis of obesity and the metabolic syndrome.

Sphingosine kinase and sphingosine 1-phosphate in cardioprotection

Activation of sphingosine kinase/sphingosine 1-phosphate-mediated signaling has emerged as a critical cardioprotective pathway in response to acute ischemia/reperfusion injury. Application of exogenous sphingosine 1-phosphate (S1P) in cultured cardiac myocytes subjected to hypoxia or treatment of isolated hearts either before ischemia or at the onset of reperfusion (pharmacologic preconditioning or postconditioning) exerts prosurvival effects. Synthetic congeners of S1P mimic these responses. Gene-targeted mice null for the sphingosine kinase 1 isoform whose hearts are subjected to ischemia/reperfusion injury exhibit increased infarct size and respond poorly either to ischemic preconditioning or to ischemic postconditioning. Measurements of cardiac sphingosine kinase activity and S1P parallel these observations. High-density lipoprotein is a major carrier of S1P, and studies of hearts in which selected S1P receptors have been deleted implicate the S1P cargo of high-density lipoprotein in cardioprotection. These observations have considerable relevance for future therapeutic approaches to acute and chronic myocardial injury.

Sphingosine-1-phosphate as a mediator of high-density lipoprotein effects in cardiovascular protection

Sphingosine-1-phosphate (S1P) has gained special attention in the high-density lipoprotein (HDL) field because HDL is the most prominent plasma carrier of S1P and because the S1P content of HDL may be responsible for many of the pleiotropic functions of HDL. This revelation has come from the evidence that HDL employ S1P receptors and signalling pathways to implement several HDL-ascribed biological effects as diverse as endothelial nitric oxide production, vasodilation, survival, and cardioprotection. This review focuses on HDL effects that are completely or partially mediated by the S1P content of the HDL particle and differentiates them from genuine HDL effects that are S1P-independent. In addition, the functional properties of 'free', HDL-unbound S1P are sometimes different from or even contrary to those of HDL-associated S1P. The nature of the physical interactions between HDL and local and systemic S1P production will be discussed as well as their consequences for organ function. Finally, we will elucidate the potential benefits and limitations of S1P analogues as a new class of functional HDL mimetics for cardiovascular therapy.

HDL and its sphingosine-1-phosphate content in cardioprotection

Increasing evidence suggests that High-density lipoproteins (HDL) are a direct cardioprotective agent in the setting of acute myocardial ischemia/reperfusion injury, and that this cardioprotection occurs independently of their atheroprotective effect. Studies on the involved mechanisms have revealed that the biologically active HDL-compound sphingosine-1-phosphate (S1P) is responsible for the beneficial effect of HDL on the myocardium. There appears to be an intricate interplay between known preconditioning agents and components of the S1P synthesis machinery in the heart, which makes S1P signalling an attractive downstream convergence point of preconditioning and cardioprotection at the level of its G protein-coupled receptors. While local S1P production has been known to protect the heart against ischemia/reperfusion injury and to mediate preconditioning, systemic S1P supply via HDL adds a novel aspect to the regulation of cardioprotection. Thus the S1P-content of HDL may serve both as a potential cardiovascular risk marker and a novel therapeutic target. Strategies for short-term "acute" HDL elevation as well as S1P analogues may prove beneficial not only in the high-risk patient but also in any patient at risk of myocardial ischemia.